The treatment and detection of cancer via photodynamic therapy (PDT) is one of the most promising and exciting applications of visible wavelength diode lasers. In PDT, a laser is used to initiate a photochemical reaction that causes cancer cell necrosis. Since early trials in 1977, more than 5000 patients world wide have received PDT treatment with promising results. One of the most attractive features of PDT is that the process is specific to cancer cells and healthy cells are essentially unaffected. Many studies have concluded that excited metastable oxygen O{sub 2} (a{super 1}delta) is the active species in the cancer cell destruction. Dosimetry is extremely important, and at the present time there is no reliable technique for real time monitoring of the O{sub 2}[a{super 1}delta] during the treatment process. Physical Sciences Inc. (PSI) proposes to demonstrate an optically based, compact, and ultra sensitive diagnostic for singlet oxygen that is based on pulsed diode lasers and the fiber optic delivery systems that are used to direct the laser light onto the cancer cells during the treatment. In Phase I, we will show feasibility of our approach by detecting singlet oxygen produced in both solution and in tissue samples irradiated by diode lasers. Imaging of irradiated samples with a medical imaging system will also be explored.